Not applicable.
Not applicable.
1. Field of the Invention
The present invention relates to materials prepared from metallurgical powders. More particularly, the present invention relates to materials prepared from metallurgical powders and which may exhibit wear, oxidation and corrosion at elevated temperatures.
2. Description of the Invention Background
Exhaust gas recirculation or xe2x80x9cEGRxe2x80x9d systems are a standard part of the pollution control equipment in automobiles. EGR systems function by re-circulating a controlled amount of high temperature exhaust gas back into the incoming fuel/air mixture. This increases the temperature of the combustion mixture and thereby reduces the amount of undesirable gases emitted to the atmosphere. FIG. 1 illustrates a conventional EGR system generally referred to as 10. The system 10 includes a valve 12, a valve guide 14, and an exhaust gas inlet 16. The inlet 16 opens into intake 18 that communicates with and transfers fresh air to a piston cylinder 20. The amount of exhaust gas that enters the intake 18 and passes to the cylinder 20 with the fresh air is controlled by the opening and closing of the valve 12 by solenoid 22.
EGR systems operate at elevated temperatures, typically 800-1400xc2x0 F. (427-760xc2x0 C.), and the valve guide and valve must be made of materials that are resistant to corrosion at high temperatures. These parts also must exhibit some degree of wear resistance. As such, the valve guides have been made from stainless steel-base materials formed by pressing and sintering metallurgical powders. Materials made by press and sinter techniques are generically known as powder metals.
The temperature at which EGR systems operate has been increasing. Initial EGR systems operated at about 800xc2x0 F. (427xc2x0 C.). At that temperature, valve guides made of copper-base powder metal alloys containing solid lubricants such as graphite proved satisfactory. More recently, EGR temperatures have been increased to about 1200xc2x0 F. (649xc2x0 C.), with long-term plans to increase operating temperatures to the 1500xc2x0 F. (816xc2x0 C.) range. At 1000xc2x0 F. (538xc2x0 C.) and above, valve guides produced from copper-base materials do not provide satisfactory performance. Instead, certain more costly stainless steel composites have been used. Those composites include stainless steel-Triballoy composites.
Triballoy is a semi-metallic having excellent wear properties and is available from Stellite Corp., Goshen, Ind. It is produced in various forms, including the following alloys (contents shown in weight percentages):
Triballoy shows good wear, corrosion, and oxidation resistance at 1000-1400xc2x0 F. (538-760xc2x0 C.). The material, however, is expensive and difficult to machine. Certain known stainless steel-Triballoy powder metal composites used to make EGR system valve guides include MPIF 316L stainless steel powder and about 20% by weight of Triballoy. Although the properties of this composite material make it highly suitable for valve guide applications, a distinct disadvantage is its high cost.
Accordingly, it would be advantageous to provide an economical powder metal material having high temperature corrosion resistance and wear resistance suitable for EGR system valve guide and other applications.
The present invention is directed to a method of forming a powder metal material or article having good high temperature corrosion resistance and wear resistance. The method includes the steps of molding a compact from a metallurgical powder, and then sintering the compact. The metallurgical powder includes at least one of a stainless steel powder and a low-chromium steel-base powder, and also includes about 0.5 to about 15 weight percent of glass powder. Preferably, the sum of the weights of any stainless steel powder and low-chromium steel-base powder in the metallurgical powder is greater than 50 percent of the entire weight of the metallurgical powder. The metallurgical powder may further include about 0.5 to about 15 weight percent, and preferably includes about 3 to about 10 weight percent, molybdenum.
The present invention also is directed to a method of forming a powder metal material or article including the step of molding a compact from a metallurgical powder including at least one of a stainless steel powder and a low-chromium steel-base powder, about 3 to about 15 weight percent molybdenum, and about 1 to about 15 weight percent of nickel-base alloy powder. The compact is sintered in a subsequent step. Preferably, the sum of the weights of stainless steel powder and low-chromium steel-base powder in the metallurgical powder is greater than 50 percent of the entire weight of the metallurgical powder. The nickel-base alloy powder preferably is at least one powder selected from nickel-base brazing powder and nickel-base wear alloy powder.
The sintered compacts produced by the above method of the invention may be subjected to a post-sinter treatment that includes: sizing the sintered compact; impregnating the compact with a suspension including a solid lubricant in a liquid carrier; and then heating the compact to substantially remove the carrier from the compact and provide a deposit of the solid lubricant on the compact. Alternatively, the post-sinter treatment includes: coating and/or impregnating the compact with a suspension including at least one solid lubricant in a liquid carrier; heating the sintered compact to substantially remove the carrier from the compact and provide a deposit of the solid lubricant on the compact; and sizing the compact.
The present invention is further directed to metallurgical powders useful in the methods described herein and to materials and articles made by the methods of the present invention. Such materials include, but are not limited to powder metal materials including a matrix comprising at least one of stainless steel and low-chromium steel, and 0.5 up to 15 weight percent of glass particles suspended in the matrix, wherein the weight percentage is based on the total weight of the material. Articles within the present invention include, but are not limited to, valve guides for internal combustion engine EGR systems, valve seats, exhaust system components, combustion chambers, other combustion engine parts subjected to high temperature (for example, 800-1600xc2x0 F. (427-871xc2x0 C.)), and chemical industry valve and corrosion parts produced using the methods of the present invention.
Certain materials within the present invention that the present inventor has evaluated exhibit good corrosion resistance and wear resistance at high temperature. It also is believed that certain materials within the present invention that the present inventor has evaluated may be produced at lower cost than conventional stainless steel/Triballoy material currently used in valve guide applications for combustion engine EGR systems.
The reader will appreciate the foregoing details and advantages of the present invention, as well as others, upon considering the following detailed description of embodiments of the invention. The reader also may comprehend additional advantages and details of the present invention upon carrying out or using the invention.